This proposal employs model mouse systems to address fundamental questions regarding the mechanism of long-term humoral immune response to pathogens. Long-lived plasma cells (PCs) in bone marrow (BM) are responsible for establishing protective life-long humoral immunity. Fully understanding differentiation mechanism of long-lived PC is critical for advanced vaccine development. We generated IA-B mice that lack MHC-II on about 95% of all B cells due to B-cell-restricted deletion of a loxP-flanked iab neo allele by the cd19cre Cre recombinase transgene. Upon immunization with a T cell dependent antigen, a small number of antigen-specific MHC-II+ B cells in IA-B mice dramatically expand to differentiate into GC B cells and make normal level of B220+ CD38+ memory B cells. However, these memory B cells lose MHC-II expression later because of on-going deletion of MHC-II by the cd19cre transgene, thus providing a unique system to study the role of MHC-II-dependent antigen presentation by memory B cells in humoral immunity. In association with loss of MHC-II on memory B cells, IA-B mice showed impaired affinity maturation in long-lived PCs. The hypothesis proposed here is that differentiation of memory B cells into long-lived PCs requires MHC-II dependent antigen presentation. The specific aims to test our hypothesis are: 1) determine the intrinsic requirement MHC-II in memory B cells function, by in vitro culture of purified VH transgenic MHC-II positive and MHC-II negative memory B cells in the presence of various stimulation conditions simulating T cell dependent or T cell independent stimulation and 2) determine the role of MHC-II in PC differentiation pathway of memory B cells, by adoptive transfer of purified VH transgenic MHC-II positive and MHC-II negative memory B cells into antigen-experienced or un-experienced recipient mice. These studies will establish whether MHC- II is required for memory function and whether T cells direct memory B cell differentiation to PC. This will form the basis for further investigations to elucidate the molecular mechanism and 'checkpoints' in the pathway of long-term humoral immunity. The outcome will be valuable for effective vaccine development as well as treating autoimmunity associated with PC differentiation of self-reactive memory B cells. Vaccination generates a small fraction of resting-memory B cells and long-lived plasma cells secreting protective antibody. This study will test whether differentiation of memory B cells to long-lived plasma cells requires MHC-II, a very important molecule for interacting with T cells. This study will further examine how this MHC-II molecule is important for memory B cell function in order to develop better vaccination. [unreadable] [unreadable] [unreadable]